Translucent electrosensitive recording sheet



Feb. 25, 1964 H. c. HILLS ETAL 3,122,448

TRANSLUCENT ELECTROSENSITIVE RECORDING SHEET Filed Sept; 28, 1960MOVEMENT -8 I I I I I I OPAQUE SEMICON- 4 DUCTIVE FUSIBLE 3 COATING 2TRANSLUCENT METAL LAYER TRANSPARENT CARRIER SHEET INVENTORS HENRY C.HILLS BY JOHN E RICE Mina/# 3 M -f' KIA/EMA ATTORNEYS 3,122,448 PatentedFeb. 25, 1964 Ofitice Our invention relates to a sheet material forrecording or the like capable of being marked directly by the passage ofan electric current, and particularly to a novel construction of suchsheet material from which electrically inscribed marks or traces may bereadily copied by direct positive reproduction processes.

Electrosensitive or thermosensitive recording sheet ma terialsheretofore known have generally comprised a dark base sheet bearing asurface coating which is initially opaque. In use, the surface coatingis burned, decomposed, fused or disintegrated in the area immediatelyadjacent an applied electrode or heated stylus to reveal the dark basesheet.

Where the eifect on the coating is produced by an are or current ofelectrons, the carrier or base sheet is usually made electricallyconductive by coating or impregnation with carbon black or metallicparticles, and the sensitive coating may also be of an electricallyconductive nature.

In thermosensitive recording media, for example, of the type disclosedby Rosenthal in United States Patent No. 2,739,909, a thermosensitivecoating is provided which is initially opaque, but which is fused totransparency or translucency by a heated stylus to expose the dark,

background of the opaque base sheet.

Regardless of the specific combination of base sheet andelectrosensitive or thermosensitive coating involved, the recordingmaterials of the prior art are of opaque nature, and may therefore beclassed as exhibiting light reflective contrast between the trace andbackground when viewed at the marking surface, but as exhibiting notransparent contrast between the trace and background because of theoverall opacity of the sheet. Reproduction of such opaque-marked papersis not possible by direct positive processes which require transparentcontrast, such as the diazo process, and of necessityexpensivephotographic processes must be relied upon for copying. Accordingly, aninexpensive recording sheet material capable of providing a recordsuitable for direct positive reproduction would be highly desirable, andit is the primary object of our invention to provide such a material.

Briefly, our invention is based on the discovery that dry electricallysensitive sheet materials capable of exhibiting the transparent contrastnecessary for direct positive reproduction may be prepared by combininga translucent or semi transparent and highly conducting base sheet witha coating of any of the known thermosensitive coating compositions inwhich particles of electrically conducting material have been dispersedto make the coating behave as a semi-conductor. With this construction,when an electrode of one polarity is brought into contact with the basesheet and an electrode of the opposite polarity is traced across thesurface of the coating, an electric current will fiow through thecoating from the trace electrode,

heating the coating in the vicinity of the trace to fuse the coatinginto transparency along the path followed by the trace electrode. Thus,a transparent trace record is provided which is suitable for directpositive reproduction.

Our invention will best be understood by reference to the accompanyingdrawing, in which the sole figure is a schematic diagram, partly incross-section, showing an exaggerated cross-sectional view of a sheet ofmaterial constructed in accordance with the preferred embodiment of ourinvention, and the electrical circuit ofa conventional recording deviceassociated therewith.

Referring now to the drawing, we have shown, greatly exaggerated inthickness, a sheet of recording material generally designated 1consisting of a base sheet comprising an optically transmissive carriersheet 2 bearing an electrically conducting and optically transmissivelayer 3, and a semi-conducting coating 4 on the base sheet, each ofwhich will be described in further detail below. As indicatedschematically, sheet 1 is adapted to be disposed on the platen 5 of aconventional recording instrument. A stationary electrode 6, which isgrounded as shown, is provided with a conducting wheel 10 which isdisposed to press sheet 1 against the platen with sufficient force tosomewhat compress the semi-conducting 4, as indicated in the drawing,over an area that is relatively large with respect to the area ofcontact of the trace electrode 7, to be described. In this manner, thedielectric strength of the semi-conducting coating 4 is sufiicientlyreduced to effect a relatively low resistance connection between theconducting layer 3 and electrode 6, the current density being reduced byits distribution over the relatively large area presented to coating 4by contact wheel 10. Alternatively, other methods of completing acircuit between stationary electrode 6 and the conducting layer 3 couldbe employed, if so desired, without departing from -the scope of ourinvention. For example, wheel 10 could be omitted and a broad contactinghead could be formed on electrode 6, or a narrow head could be employedif pressed hard enough into sheet 1 to sufficiently reduce theresistance between electrode 6 and conducting layer 3. "Or, if desired,the semi-conducting coating 4 could be omitted at an edge of sheet 1, toexpose conducting layer 3 to direct contact with electrode 6 or anequivalent electrode with an elongated contacting edge. In a manner thatwill be well understood by those skilled in the art, a traceelectrode 7,which is adapted to be actuated by the movement 8 of the recordinginstrument, is disposed to move over the surface of coating 4 todescribe a trace to be recorded.

As indicated, trace electrode 5 is returned to ground through a suitablesource of voltage, schematically indicated as a battery 9. The use of adirect current source, with the polarity indicated in the drawing, ispreferred, since we have found that a smooth trace with less erosion ofthe trace electrode will result from this arrangement. However, it iswithin the scope of our invention, in its broader aspects, to employ analternating source of voltage, and in this event the platen may begrounded to serve as the stationary electrode, replacing electrode 6,and the capacitance between the platen and conducting layer 3 willcomplete the electrical circuit. It will be appreciated that the detailsof the recording instrument with which our novel recording sheet is em--ployed do not form a part of our invention, and accord ingly, we haveonlyshown those portions of a conventional instrument which directlycooperate with the recording sheet and serve to illustrate its use.

snags-as Turning now to the detailed construction of theelectrosensitive recording sheet of our invention, carrier sheet 2 canbe made of any suitable thin, relatively strong material, which issufliciently transmissive to light to serve as a base for direct copyingprocesses. For ex ample, transparent plastics such as Mylar polyesterfilm, or translucent materials such as glassine or the like, can beemployed. Preferably, the conducting layer 3 comprises a metallicdeposit of aluminum on sheet 2, sufii ciently thin to be opticallytransmissive to an extent permitting the exposure of underlying diazotype copy papers, layer 3 forming a half-mirror surface on the carriersheet 2. In addition to aluminum, it is obvious that any other suitableconductive metal could be employed, such as zinc, copper, gold, silver,or the like.

Since the sole purpose of sheet 2 is to form a strong support for thevery thin, relatively weak layer 3, it can be omitted, if desired, byemploying a conductive base sheet combining the qualities of relativelyhigh conduc* tivity, strength, and optical transmissiveness. In particular, we have found that in addition to metallic coatings, various typesof electrically conductive glass surfaces, such as Libby-Owens-Ford Sl-Eglass coatings, may be employed without departing from the scope of ourinvention in its broader aspects.

As generally described above, the electrosensitive coat ing 4, which incombination with the transmissive elec trically conductive layer 3results in a direct positive reproduction master when electricallymarked, must be of a type which is opaque to ultra violet light beforebeing acted upon by an electric current but which subsequently becomestransparent in the area of the mark or trace. The coatings abovereferred to as being disintegratable or removable by the reaction toelectric current would serve in our novel combination, but we prefer touse a coating which is less destructible in its reaction and which isless likely to produce undesirable rubble that would collect on thestylus or trace electrode of a mark ing instrument. Specifically, Weprefer to employ an opaque electrosensitive coating which will fuse totransparency by the passage of an electric current. The ex act degree ofopacity of the coating is not critical, so long as it is sufficient togive adequate contrast with the backing material by transmitted light.As an example of a coating suitable for this purpose, in accordance withone embodiment of our invention, We may employ an electrosensitizedblushed lacquer. Thus, for example, cellulose acetate may be dissolvedin a solvent mixture of acetone and Water, to which has been added astrong electrolyte such as nickel nitrate, sodium thiocyanate, so diumchloride, or the like, and the solution coated on the conducting layer3. On drying, the acetone evapo rates the more quickly, and causescellulose acetate par ticles and included electrolyte particles toprecipitate out, the resultant film being blushed as it is termed, andin this state appearing as a White opaque coating. Due to the includedelectrolytes, the coating behaves as a semi-conductor, and in a thincoating of less than 3 mils, and preferably of the order of 1 mil orless, will conduct sufficient current upon the application of a moderatevolt age of 300 volts or less to heat the point directly adjacent thetrace electrode to fusion, in which state the material of the coating isessentially transparent.

A second, and preferred, coating which may be em ployed comprises aplastic composition including a mul tiplicity of enclosed microscopic orsubmicroscopic voids throughout its volume beneath the outer surfacethereof, and including dispersed particles of a strong electrolyte or ofa finely divided metal. Such a plastic coating, apart from the voids, isessentially continuous and homogene ous, and the film as a whole isopaque because of its heterogeneous physical structure, due to thevoids. This coating may be prepared in the manner fully described inUnited States Letters Patent No. 2,739,909 to Rosenthal, with theexception that electrically conductive particles are incorporated in thecoating during its manufacture. While this may be done in numerous Waysknown to the art, one simple method is to dissolve in the water phase ofthe coating emulsion a suitable electrolyte, for example, 25% of sodiumthiocyanate by weight. Of course, the electrolyte could be dissolved ineither the water phase or the organic phase, and in place of theelectrolyte, metallic particles could be dispersed in both phases. Theresult in any event will be an opaque semiconducting coating which canbe fused to transparency by an electric voltage applied across it. Whilevarious theories may be advanced to explain the semiconductive nature ofthe coating, and we do not wish to be bound by any particular theory, webelieve that the mechanism involves arcing over short paths from oneconductive particle to the next, with a net lowering of the breakdownpotential or an effective decrease in the resistance of the coating. Itwill be obvious that the required voltage can readily be adjusted byadjusting the thickness of the film and the concentration of electrolyteor other conductive material added to it. Since these matters are wellwithin the scope of the designer, and can be determined by calculationor by routine experimentation without the exercise of the inventivefaculties, it is believed unnecessary to describe them in furtherdetail.

In the preferred embodiment of our invention, a semiconducting opaquecoating is employed which is fused to transparency by the passage of anelectric current. However, it is Within the scope of our invention inits broader aspects to employ other coatings known to the art, whichrespond to the passage of an electric current by vaporizing ordisintegrating in the path of the trace electrode to expose theunderlying base sheet to the passage of light.

While we have described various embodiments of our invention in detail,many changes and variations will become apparent to those skilled in theart upon reading our description, and such can obviously be made withoutdeparting from the scope of our invention. Accordingly, we do not wishto be limited to the details shown, but only by the spirit and scope ofthe following claims.

Having thus described our invention, what We claim is:

1. A normally opaque recording sheet capable of being marked by thepassage of an electric current, comprising an optically transmissiveelectrically conducting base, a coating on said base of an opaquematerial fusible to transparency upon heating, and electricallyconducting particles dispersed in said coating in an amount suificientto render said coating conductive.

2. An electrosensitive normally opaque recording sheet, comprising, incombination, an optically transmissive carrier sheet; a thin translucentlayer of metal deposited on said carrier sheet; and an electrosensitivecoating on said sheet comprising fusible plastic, containing sufficientvoids to be opaque to light, and having particles of an electrolyticsalt dispersed therein in an amount sufiicient to render said coatingconductive.

3. A normally opaque recording sheet, comprising a sheet of translucentelectrically conductive material and a coating on said sheet of anopaque semiconducting material fusible to transparency by the passage ofan electric current through said coating whereby an opticallytransmissive mark is formed on said sheet.

4. An electrosensitive normally opaque recording sheet, comprising anoptically transmissive and electrically conductive base sheet and anopaque conductive coating on said sheet capable of being renderedoptically transmissive by the passage of an electric current throughsaid coating whereby an optically transmissive mark is formed on saidsheet.

5. A normally opaque recording sheet comprising, a base sheet ofoptically transmissive sheet material and a layer on said sheet of metalsufficiently thin to be optically transmissive, and an opaque coating onsaid sheet of a material reactive to the passage of an electric currentthrough said coating to render said coating optically 5 transmissive andthereby to permit the passage of light through the sheet.

6. An electrosensitive normally opaque sheet, compris ing an opticallytransmissive electrically conductive base sheet and an opaque coating onsaid sheet of a material reactive to the passage of an electric currentthrough said coating to render said coating optically transmissive andthereby to permit the passage of light through the sheet.

References Cited in the file of this patent UNITED STATES PATENTS2,306,525 Cummings Dec. 29, 1942 5 Dalton Mar. 16, 1943 Quick May 4,1943 'Engler et a1. Apr. 18, 1944 Roddin et al. Dec. 6, 1955 RosenthalMar. 27, 1956 Scott July 16, 1957 Newman et al Oct. 22, 1957 VanderNVeelMar. 1, 1960 Ressler Feb. 14, 1961 Grupe Dec. 26, 1961 Dalton Mar. 13,1962

1. A NORMALLY OPAQUE RECORDING SHEET CAPABLE OF BEING MARKED BY THE PASSAGE OF AN ELECTRIC CURRENT, COMPRISING AN OPTICALLY TRANSMISSIVE ELECTRICALLY CONDUCTING BASE, A TRANSPARENCY UPON HEATING, AND ELECTRICALLY CONDUCTING PARTICLES DISPERSED IN SAID COATING IN AN AMOUNT SUFFICIENT TO RENDER SAID COATING CONDUCTIVE. 